PHF in Oklahoma

The death of at least one Oklahoma horse has been definitively linked to Potomac horse fever (PHF), a disease rarely found in the state, and two of her stablemates likely died of the same illness. The horse manager at the farm with the confirmed case said that 11 other horses in the area have died following similar clinical signs and debilitation, but that blood and tissue samples from those horses were not submitted for diagnosis.

The actual number of PHF cases is not known since it is a disease not typically seen or reported by equine practitioners in Oklahoma.

Potomac horse fever's causative agent, a bacterium named Neorickettsia risticii, has been linked to parasites of freshwater snails. The parasites are called cercariae, and they also infect the larvae of mayflies and caddis flies in fresh water. When the infected fly larvae mature into infected adult flies, they can be ingested by horses which inadvertently consume the insects while grazing or eating feedstuffs. Horses kept near fresh-water streams or ponds are more likely to be at risk for getting the disease because of the close proximity of the aquatic insects. There is a PHF vaccine, but its efficacy has been questioned by veterinarians.

Rocky Carroll, horse manager at Black Fox Ranch in Cherokee County, Okla., reported that three horses from his farm died with similar clinical signs between June 29 and Aug. 22. The first horse died within 48 hours of clinical signs appearing; the second horse was treated at Oklahoma State University and was euthanized after about 30 days of treatment. He said that the ranch is about a mile from the Illinois River.

Mike Sheets, DVM, of Northside Animal Clinic in Stillwell, OK, treated the third horse from Black Fox Ranch. Sheets first saw the horse at the farm Monday night, Aug. 11, and the horse was admitted to his clinic on Aug. 12. The horse was "initially depressed, acutely febrile, had a pounding digital pulse, a lack of gastric motility (no gut sounds), but no lameness," he said. The horse had a low white blood cell count and developed edema in the limbs. "I started treating with tetracycline IV, and it was about 72 hours before it developed laminitis (what often leads to the demise of horses with PHF). Then after the laminitis, it started showing pain in its feet and broke with diarrhea, but only for about 24 hours duration." The diarrhea subsided, but the laminitis continued. The horse was euthanized late Aug. 22.

Also on Aug. 22, Sheets received positive polymerase chain reaction (PCR) test results from the University of California, Davis. "The U Cal-Davis turnaround is pretty remarkable," he said. "I sent (fecal and blood samples) in, they got it on Friday, and they called and left me word Friday night." Sheets was already treating the case as if it were PHF since he had heard of other suspect cases in the area. John Madigan, DVM, MS, Dipl. ACVIM, of UC Davis, has been involved in PHF research for nearly 10 years, and his laboratory performs the PCR testing The Horse has spoken to Madigan about PHF in Oklahoma--stay tuned for more information.) Sheets said that colon tissue from the first Black Fox case of PHF will be sent to Madigan for retrospective testing.

According to Jack Carson, spokesman for the Oklahoma Department of Agriculture, Food, and Forestry, "We have had two horses that have either died or been put down from (PHF), and there’s another one that is being treated, all in the Eastern part of the state." Carson wasn’t able to verify in which counties the horses were located.

Potomac horse fever in Oklahoma is "extremely rare, in fact I was talking to our state vet about it and he said he hadn’t heard of it in over two years," added Carson. Sheets said that he doesn’t recall any PHF cases in the state in recent years, but added that the farm where the horses were found was "on a river bottom."

Already this year, "Several horses along the Illinois River have died prior to ours with reasons to suspect PHF," said Carroll.

Retrospectively, Sheets believes that a horse that he treated in July might have been another case of PHF. "It had diarrhea several days before it ever developed laminitis, and it cultured negative for salmonella."

As with discovery of any disease symptoms, Carson said that horse owners should contact their veterinarians. If the veterinarian makes the initial diagnosis of PHF or finds anything else "out of the ordinary," then the vet will contact a state or federal official veterinarian.

Catching it early is of utmost importance, however. Sheets looked to other veterinarians who had treated PHF cases in recent years and learned from their experiences--"You need to treat it in the first 72 hours of clinical signs or you’re not going to have much luck," said Sheets.

About the Author

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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